Effect of Gasoline Fuel Additives on Combustion and Engine Performance

2.1.2
 
Combustion Improvers 
Combustion improvers (CI) are in broad terms additives that improve fuel’s 
combustion characteristics. In gasoline fuels, CIs primarily improve a fuel’s octane 
number which in practice relates to its knock resistance. As such, CIs in gasoline are 
often also called anti-knock additives. Gasoline combustion is based upon a single 
source of ignition that initiates flame kernel growth and the subsequent flame 
propagation. Fuels that auto ignite prior to this can cause violent combustion that can 
raise the pressure beyond the designed values and damage the engine. According to 
Zhen et al. [41], knocking over a prolonged period causes damage mainly to piston 
rings, cylinder head and piston crown erosion, piston melting and increases the 
emissions levels. 
Knocking in gasoline IC engines occurs when enough energy is present to 
cause stable molecules within the fuel to break down without an external energy 
source. This usually occurs within the air-fuel mixture in a cylinder that is reached 
latest by the propagating flame front, called the end gas [42]. Due to increasing 
pressure and temperature conditions as a result of piston movement and flame 
propagation, the end gases can reach an energy state high enough to initiate 
combustion reactions which produce highly reactive radicals before being consumed 
by the propagating flame, resulting in knocking. Chain initiating reactions are 
followed by propagation and chain-branching reactions, where the aforementioned 
radicals react to form products and new radicals. In the propagation reactions, the 
number of radicals consumed is equal to that produced while in chain branching 
reactions more radicals are produced than consumed. The final termination reactions 

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2.1 Fuel Additive Review 
32 
take place when fewer radicals are produced than consumed. This process will 
continue until all unstable radicals are converted into stable reaction products.
CI anti-knock additives are molecules that break down at lower temperatures 
than knocking conditions and provide radicals that either consume chain initiating 
radicals or cause chain debranching, thus preventing auto ignition [43]. However, as 
Benson [43] further explains, the additives are often limited in the temperature range. 
A shift in a temperature conditions can initiate a different path of reactions that could 
in fact support chain initiating reactions. 
Initially an organometallic additive Tetraethyl Lead was used but starting with 
legislation changes from 1996 up to 2005, lead anti-knock additives have been banned 
from being sold within the EU. This is due to an increase in concentrations of lead in 
the bloodstreams of people living in urban environments which has a negative effect 
on the human intellectual development [44]. It has been reported that an increase of 1 
µg/dl blood lead level can decrease the average intelligence quotient (IQ) by 0.25 
points [45]. Although this effect can be considered low, Rosen [46] argues that it 
applies to a very large proportion of population and affects the number of people with 
exceptional IQ score. This has caused a market change towards lead replacement 
additives but has also indirectly created grounds for an increase in the use of diesel 
fuel.
According to Dabelstein et al. [47], other metallic compounds have been used 
as anti-knock additives but can exhibit similar shortfalls to lead compounds. 
Furthermore, ashless (non-metal containing) nitrogen based aniline additives can be 
used, though, their effectiveness is often reduced in small quantities and higher treat 
rates are economically unviable in comparison. Moreover, it has been found that fuel 
bound Nitrogen compounds have a capability to increase NO
x
emissions [48, 49]. 
Ickes et al. [50] argue, however, that any increases in emissions levels due to additives 
are offset by the benefits offered by using the additives. In some countries, oxygen 
containing compounds, such as ethers are used instead of CI additives, although the 
quantities involved mean they are not considered as fuel additives but fuel 
components. 
Further improvements can be seen by using metallic additives that work based 
on a catalytic effect, although the use has been stopped in developed countries. May 
[51] explains the catalytic effect of iron and magnesium based CI additive with 
excitement of outer layer electrodes. At the start of combustion these electrodes 

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2.1 Fuel Additive Review 
33 
acquire energy states higher than that of surroundings. They then release this energy 
to reach a degenerate state, thus maintaining the required reaction activation energy 
level and reducing the effect of quenching or other thermal losses that would hinder 
completion of combustion reactions.
Due to possible metal phase sintering under high temperature conditions 
during combustion but also financial viability, typical treat levels of organometallic 
catalytic CI fall below 1,000 ppm [52] although effects can be seen from treat levels 
as low as 0.005 ppm [53]. Lyons and McKone [54], using organic compounds of 
selenium, animony, arsenic, bismuth, cadmium, tellurium, thallium and tin, argue that 
concentrations of at least 10-400 ppm need to be reached before instantaneous effects 
can be seen. However, they note that in time, with build-up of catalytically active 
deposits, it is possible making lower treat rates of additive effective. This conclusion 
is further supported by field tests by May and Lang [55] who saw a delay of a ‘few 
days’ before full effect of additive was experienced. The additives in this case included 
combinations of magnesium and iron and pure iron. Typical additives used as catalytic 
CI are alkali or other metal salts although it has been noted that some metal containing 
detergents can also act as CI [14, 56, 57].
In diesel fuels, CIs improve the fuel’s cetane number, which constitutes as 
ignition quality. Unlike in gasoline fuels, diesel combustion relies upon auto-ignition. 
The higher the cetane number the easier the fuel will ignite and shorter the ignition 
delay (time between SOI and start of combustion). CI are unstable molecules that can 
easily break down at relatively low temperatures to provide free radicals in combustion 
reactions to enable faster conversion of a hydrocarbon fuel into CO
2
and H
2
O [58].
Typical diesel CIs include organic nitrates and organic peroxides, with the most 
common compound being 2-ethylhexyl nitrate (2-EHN) [59, 60].
CI can reduce emissions in diesel engines. May and Lang measured a reduction 
in NO
x
emissions of over 75 %. They contributed the effect to lower combustion 
temperature due to enhanced lean burning capabilities of the fuel. Gonzales [56] 
similarly found an advantageous effect of a CI additive with a 25.1 % decrease in NO
x
emissions. This, however, was contributed to an increased resonance radiation from 
the core of the flame which would result in a cooling effect along the outer edges of 
the flame. A further explanation is proposed by May [61], whereby the enhanced 
reaction speeds reduce the time available for NO
x
formation. May & Lang and 
Gonzales further found a decrease in other controlled emissions (see Section 2.4.4) 

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2.1 Fuel Additive Review 
34 
and improved fuel consumption. Higgins et al. [10] showed that 4,000 ppm of 2-EHN 
additive in low cetane number fuel reduced the ignition delay for low pressure and 
temperature conditions compared to equivalent high cetane number fuel. Furthermore, 
the increased nitrogen from the additive composition was observed to increase the 
NO
x
emissions. 
Although cetane improvers are designed to promote auto ignition in diesel 
combustion, in some cases adding these additives to gasoline fuel has been found to 
be advantageous. Colucci et al. [11] show that using 2-EHN in gasoline, the ignition 
properties of the fuel can be significantly improved. They found that at 100 ppm treat 
rate, misfires during the cold start period have been completely removed. Moreover, 
it is claimed that the additive would enable reduced cycle-to-cycle variations during 
normal running. However, no mention is made of the limiting engine operating 
conditions where auto ignition could start occurring. According to Aradi and Roos 
[62] 2-EHN only survives at temperatures up to 625 K. They argue that improvement 
in engine efficiency could be achieved if additives that dissociate near the ignition 
temperatures of around 800 K were utilised. Furthermore, Morsy [63] displayed the 
usefulness of diesel orientated CI as tools to control combustion in homogeneous 
charge compression ignition (HCCI) engines where gasoline-like fuels are utilised and 
Becker [64] demonstrated the use of cetane improvers in high octane fuels to be 
beneficial for multi fuel engines. 
Due to similar nature of base fuel and their combustion process, CI are suitable 
for most liquid fuels including gasoline, diesel, kerosene and others [65, 66]. However, 
some CI additives are fuel insoluble and can contribute towards fuel system deposits. 
As such, they are often used in conjunction with carrier fluids. Kitchen [67] uses small 
quantities of naphtha and polyalphaolefin synthetic oil for such purpose along with 
combustion enhancing manganese linoleate. More detailed description of carrier fluids 
will be offered in Section 2.1.6.